Simulating Two-way Interactive Vegetation-Biophysical Processes and Atmospheric- Mesoscale Circulations During 2001 Santarem Field Campaign Using SiB-RAMS Model

The Simple Biosphere Model Version 2.5 (SiB2.5) is coupled with the Colorado State University Regional Atmospheric Modeling System (RAMS) to study the two-way interactions between the land surface and atmosphere during the dry season 2001 Santarem field campaign. The evaluations against flux tower and nearby meteorological station observations show that the SiB-RAMS is able to capture the variability in observed meteorology and CO2 concentration, as well as surface fluxes of CO2, H, and LE, during the 15- day simulation time period, 1 through 15 August 2001. The mechanically forced low-level convergence on the eastside of Tapajos" river has significant impact on observed ecosystem carbon fluxes, and is taken into account when tower fluxes are generalized to a large region. The impact of CO2 source from the Tapajos River was also examined by performing numerical sensitivity experiments with and without specifying river CO2 effluxes. The magnitude of these fluxes is 5 umol/m2/s, which is determined by boat measurement. The results show that the river CO2 effluxes modify the spatial and temporal distributions of atmospheric CO2 concentrations, especially at night. Our simulation also indicated the Amazon Basin continues to be a carbon sink even when the additional CO2 from the river to the atmosphere is accounted for. The additional CO2 source enhances carbon uptake over vegetated land on the lee side of Tapajos" river. Furthermore, a Lagrangian Particle Dispersion Model (LPDM) was used to quantify the contribution of river CO2 effluxes to tower observed variability in CO2 flux and concentration. A transect of 10 virtual towers was placed across the Tapajos River with 4-km spacing between each tower in the east-west direction. The particles were released and tracked backward in time at each tower. The influence function was calculated and integrated with the surface CO2 fluxes simulated by SiB-RAMS. The experiment demonstrates that LPDM is capable to determine when, where, and how much the river CO2 sources influence the observed CO2 at various tower locations.